I've done a lot of research into this issue and have published several magazine articles on it. Interviewed S&W personnel, cops, other writers, and ammo company executives and technicians...
I resolved not to use the light bullet .357 loads in K-frame guns. The lightest .357 bullets I shoot are 140 grains. More often, I use 158 grainers.
But for .38 Specials, the pressure level is much less. I do use 125-135 grain loads in .38's, inc. snub and three-inch guns. I'll probably try some Speer 135 grain Short Barrel .357 loads in my Ruger SP-101. They only clock some 990 FPS in a snub gun, so aren't loaded as hot as most .357 ammo. The bullet is designed to open well at moderate velocities. My three-inch bbl. will probably deliver ca. 1,040 FPS with that load. It is also the hottest .357 round that I'd willingly fire indoors without ear protection. That DOES NOT mean to use it without protection on indoor ranges. Just means that I probably won't be deafened by the 2-5 shots that I might have to fire at a burglar. Most of my home defense guns use .38 Plus P loads.
I think you'll be okay with the .38 Blazer rounds. I don't buy them because of the aluminum cases. You could just call Speer and ask their opinion. While you're at it, ask why Blazer isn't suggested for duty use. If this ammo is carried in the gun and in your belt ammo pouches for long periods, does the sealing leak? Does the alloy change characteristics or break down? Do primers remain well sealed? Or is it just a matter of the cases not being as well sealed as normal brass cases intended for duty use? Does the crimp lessen?
If you're just shooting them off at the range, I doubt that you'll have any problem. But space it out. The more you fire in a session, the hotter the gun gets, and hot metal wears at a faster rate than at normal temperatures. Or, so I was told.
Bill Jordan himself told me that he thought that most owners of K-frame .357's would fire just 10-15% .357 loads. At the time that he conceived the gun, most cops practiced with .38 wadcutters and just carried Magnums on duty.
Later court decisions mandated practicing with the work load, and this resulted in far more Magnum ammo, much of it the hot 125 grain stuff, being shot in these guns. This resulted in problems. That is why the L-frame guns were developed!
If you're a home defender or camper or hiker or occasional handgun hunter who doesn't fire thousands of .357 rounds a year, you're probably okay with a K-mag. If you shoot a lot of .357 ammo, buy a larger gun.( K-mag is my term, in case you see it copied by others. It just refers to any .357 S&W built on the medium/K frame.) It does seem to be true that Rugers are more durable in frequent use with .357 ammo, but unless you shoot a LOT, it's not a big issue.
